Definition: a point where different light rays meet, the optimum adjustment of an imaging system, or a location along a beam where the beam diameter reaches a minimum
Alternative terms: focal point, beam waist
Categories: general optics, laser devices and laser physics
Author: Dr. Rüdiger Paschotta
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In the field of optics and laser technology, the term focus occurs with several different meanings, which however are related to each other.
Focus in Geometrical Optics and Imaging
In geometrical optics, a focus is a point where different light rays meet. More specifically, this is mostly in the context of imaging, where different light rays emerging from a common point of an object and propagating through some imaging optics are considered. For example, such a focus point may be located on the surface of a focal plane array, when the imaging optics are optimally adjusted, and best images are then obtained. If the optics are somewhat defocused, the focus points for different object points may be somewhat before or after the surface of the detector, resulting in larger illuminated spots and thus less sharp images.
The noun focus can also mean the adjustment of an imaging system for best sharpness of the images – effectively placing the focal point on the appropriate plane. Focus may also refer to the object plane for which optimum sharpness is achieved (e.g. “the focus is at 30 cm distance”); note that there is usually a limited depth of field, and objects outside that range are out of focus.
A focal point is a focus under specific circumstances.
There is also the verb to focus; this means to achieve best focus of an imaging system. Autofocus means automatic adjustment of focus of an imaging system. The opposite of to focus is to defocus, i.e., to cause blurring of an image.
The wave nature of light is often not considered in this context. However, one can use wave optics to take this into account. One then finds that imaging leads to spots with a finite size in the image plane. Optimal focus, for example concerning the longitudinal position of a focal plane array, then results in spots of minimum size.
Focus of a Laser Beam
A focus of a laser beam (a beam focus or laser focus) is understood to be a longitudinal position (or a spatial region) where the beam radius reaches a minimum. This is also called a beam waist.
Note that the focus points concerning different transverse directions may be at different locations; see the article on astigmatism. Also, the focus position may depend on the wavelength as a result of chromatic aberrations.
For propagation of light in a non-absorbing medium, a focus is a point with maximum optical intensity. Indeed, the purpose of focusing a laser beam is often that sufficiently high intensities for some laser application can be achieved that way. It may also be desirable to strongly limit the exposed area, e.g. in laser micromachining.
Due to the high spatial coherence which laser light often exhibits (in ideal cases allowing for diffraction-limited beams), tight focusing (i.e., to a small spot) is possible: the focus beam radius is often of the order of the optical wavelength or even somewhat smaller. A high beam quality essentially means that a beam can be well focused.
In the focus of a beam with optimal beam quality, the wavefronts are plane. Before and after the focus, the wavefronts are curved, and that curvature is associated with the converging or diverging of the radiation. Beams with non-ideal beam quality can exhibit substantially scrambled wavefronts in the focus.
In the context of laser beams, wave optics are usually used because wave effects determine the focus spot size, which is often quite relevant for applications.
Some related terms:
- A focus position is the longitudinal or transverse position of a beam focus.
- The length of a focus can be quantified with its Rayleigh length.
- Tight (or sharp or strong) focusing means focusing light to a small spot – the opposite to mild focusing.
- The focusing distance of a focusing optical element is the distance from that element to the location of the achieved focus.
- Re-focusing a light beam means to focus it again at a further longitudinal position. For example, light emerging from an optical fiber exhibits some divergence but can be refocused to a spot at another position using a suitable lens.
In some cases, the focusing of light means somewhat more generally to manipulate it such that it gets more concentrated – even if a true beam focus (or a crossing point of light rays) is not obtained, for example because the light hits some object before it reaches the minimum beam diameter. In that sense, a focusing lens (or other optical element) is one which transforms a collimated beam (for example) into a converging beam, or more specifically a lens which is used for the purpose of focusing. The focusing essentially means a change of curvature of the wavefronts. Lenses and curved mirrors are often used for focusing or defocusing light.
See also: focal points and focal planes, geometrical optics, imaging, beam waist
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How tightly can a fiber laser beam be focused? Is it entirely dependent on MFD of the fiber or on the laser system used for the particular setup?
The author's answer:
Some fiber lasers have a diffraction-limited output, and the focusing is essentially only limited by the wavelength. Others have a spatially multimode output, and then the focusability (beam quality) is lower.
In any case, the mode area of the fiber is not relevant.